The only way I know to protect further character names or imagery in an open source game (or for any open source project) would be to use trademarks e.g. if you explicitly mention that the names and images are trademarked by you (and eventually register all of them), then you will enjoy this protection for your names and marks.
It is common for several licenses such as the Apache and BSD licenses to contain some explicit language about non-usage of names and non-endorsement but to the best of my knowledge this is not necessary because these licenses do not grant rights to reuse these names.
It is common for several open source projects (and foundations) to register their names and marks, such as GNOME which allowed to protect the IMHO abusive reuse attempt by Groupon, or Eclipse that also provides detailed guidelines for limited and proper reuse. Linux distributors such as Ubuntu and Red Hat also use trademark protections in addition to the (eventually complex) FOSS licensing of their distributions. In these later cases, redistributing a whole distro may be OK from an open source license standpoint but may not be OK from a trademark standpoint (in which case a solution is to remove the marks and graphics like CentOS did with Red Hat distros).
So in recap, the trademark protection and the free and open source licensing protection are orthogonal and both can apply and both are commonly used in conjunction by both commercially-backed FOSS projects and purely community-driven projects.
Since trademarks themselves are beyond the scope of most open source licenses, you should seek legal help and do some research to achieve whatever it is you want to achieve there. God speed!